The police on Friday arrested a 30-year-old man in connection with the murder of a 38-year-old school teacher in northwest Delhi's Bawana area, officials said.
Four persons have been arrested in the case so far, the police said.
Manjeet (38), husband of the deceased, his girlfriend Angel Gupta alias Shashi Prabha (26) and Rajeev (40), were arrested on Thursday.
The fourth accused, identified as Deepak, was Rajeev's driver for the past 10 years, the police said.
Manjeet was in a relationship with Angel and his wife, Sunita, objected to it, Deputy Commissioner of Police (Rohini) Rajneesh Gupta said.
During the investigation, Sunita's relatives handed over her diary to police in which she had written that her married life was unsatisfactory.
Manjeet's in-laws had also learnt of his illicit relationship, following which he promised to end it, Gupta said.
He and Sunita recently had a heated argument when she found out that her husband had not ended his relationship, he added.
Fed up with this, Manjeet conspired with Angel and Rajeev to eliminate his wife, the DCP said.
Deepak hired two sharpshooters who came up with a plan to kill Sunita after carrying out a reconnaissance of the area, he said.
Sunita was shot dead on Monday morning when she left her house at around 8 am. She was shot thrice, Gupta said.
(With inputs from agencies.)